Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern can be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. Manufacturing different layers of the IC often requires imaging different patterns on different layers with different reticles or masks.
As the dimensions of ICs decrease and the patterns being transferred from the mask to the substrate become more complex, defects in the features formed on the mask become increasingly important. Consequently, defects in the features formed on the mask translate into pattern defects formed on the substrate. Mask defects can come from a variety of sources such as, for example, defects in coatings on mask blanks, the mask patterning process in a mask shop, and mask handling and contamination defects in a wafer fabrication facility. Therefore, inspection of masks for defects is important to minimize or remove unwanted particles and contaminants from affecting the transfer of a mask pattern onto the substrate.